The spacing between conductive parts is important for designing electrical equipment. The spacing is designed to be sufficiently wide to avoid an electrical breakdown between conductive parts to be insulated from each other, especially in a high voltage environment. The required spacing between the conductive parts for insulation is dependent on pollution level of the environment which surrounds the conductive parts, distances of the conductive parts to other conductive parts, working voltage of the equipment, and comparative tracking index or resistance to surface tracking of material used in the equipment.
The spacing can be characterized by clearance and creepage distance according to IEC 60664. The clearance is generally defined as the shortest distance between two conductive parts measured through air. The creepage distance is generally defined as the shortest path between two conductive parts measured along a surface of insulation. The comparative tracking index (CTI) of a material is a measure of a resistance of the insulator to surface tracking, which is an electrical breakdown on the surface of the material.